Phases of clinical trials
Many clinical trials to develop new interventions are conducted in phases. In the early phases, the new intervention is tested in a small number of participants to assess safety and effectiveness. If the intervention is promising, it may move to later phases of testing where the number of participants is increased to collect more information on effectiveness and possible side effects.
Clinical trials of biomedical interventions typically proceed through four phases.
Phase I Clinical Trial
Phase I clinical trials are done to test a new biomedical intervention for the first time in a small group of people (e.g. 20-80) to evaluate safety (e.g. determine a safe dosage range and identify side effects).
Phase II Clinical Trial
Phase II clinical trials are done to study an intervention in a larger group of people (several hundred) to determine efficacy (whether it works as intended) and to further evaluate its safety.
Phase III Clinical Trial
Phase III studies are done to study the efficacy of an intervention in large groups of trial participants (from several hundred to several thousand) by comparing the intervention to other standard or experimental interventions (or to non-interventional standard care) as well as to monitor adverse effects and to collect information that will allow the intervention to be used safely.
Phase IV Clinical Trial
Phase IV studies are done after an intervention has been marketed. These studies are designed to monitor the effectiveness of the approved intervention in the general population and to collect information about any adverse effects associated with widespread use over longer periods of time.
Other Clinical Trials
Researchers may also conduct exploratory studies, sometimes referred to as ‘Phase 0 trials’ or ‘pilot studies’ that come before Phase I trials and are used to test how the body responds to an experimental drug. In these studies, small doses of the new drug are given once or for a short time to a very limited number of people.
Clinical trials of diagnostic tests are sometimes divided into exploratory phases, challenge phases and advanced phases to see how effective and how accurate the tests are.
Find a clinical trial website
Search for clinical trial registries, organisations, research networks, coordinating hubs and support groups by specific disease or condition.
Consumer guide to clinical trials
The Consumers Health Forum have developed a factsheet about clinical trials.
Questions to ask
Need help with questions to ask if you're thinking about being part of a clinical trial?
Download the Questions to Ask Fact Sheet (PDF, 253KB) and take it with you.